Switch-type probe heads, as used on coordinate-measuring machines, employ a three-point support to which a probe pin is pressed by spring force and which, with a high degree of precision, fixes the position in space of the probe pin and of its work-contacting probe ball. At the same time, this three-point support is developed as an electrical switch contact, in the manner of a normally closed switch which is actuated to open condition upon a lifting of the probe pin off any of its three points of support, in the course of a work-contacting process. Switch-type probe heads are illustratively described in German Federal Republic Patents 2,347,633, 2,712,181, 2,743,665 and in German Federal Republic OS No. 2,947,394.
It is also known that a plurality of switch-type probe heads can be mounted at the same time on the measurement arm of a coordinate-measuring machine. Thus, for example, the British commercial brochure, entitled "Three Dimensional Touch Trigger Probes MM for Measuring Machines" of Renishaw Electrical Ltd. (Ref. MM 981), describes a holder designated PH 5 which permits a simultaneous attachment of five probe heads in a star-shaped geometrical configuration. As shown in the schematic diagram of FIG. 1, the switch contacts of the plural probe pins (2a, b, c) in such a holder are connected in series to each other and to a common electronic unit (1). However, this kind of interconnection is disadvantageous, inasmuch as cable resistance and switch-contact resistance are cumulative, so that it is difficult to precisely establish the exact time of switching of a given probe head independently of the number of probe heads, particularly for situations involving a large number of probe heads. Furthermore, blind plugs (4a and 4b) which are not provided with probe heads must be provided for the connections in order to assure the operation of the arrangement.
The same commercial publication also describes a holder designated PH 7 which has two connections for probe heads, and these connections are in parallel. On this holder, however, only one probe head can be mounted, either to the one or to the other connection; otherwise, the switch contacts of plural probe heads short-circuit each other.
The above-discussed problem of cumulative contact and cable resistances can be circumvented if, as indicated in FIG. 2, the switch contacts (13a, b, c) of plural probe heads (12a, b, c) are fed separately to the electronic unit (11), and if all switch contacts are brought together via an OR-gate (14), in order to produce a common contact pulse. This solution, however, has the disadvantage that a very large number of connecting cables are then necessary, and it is not possible to employ connecting pieces and distributors in multiple branching of individual feed lines.
From Federal German Republic Patent 2,725,996, a probe head is known which has five individual switch-type probes, also in star-shape geometry. As can be noted from FIG. 9 of said patent, the switch contacts (71-75) of these five individual probes are connected to each other, in part in parallel and in part in series, by means of diodes (90/91), thus enabling automatic recognition of the individual probe which has effected a specific work-contacting procedure. In this arrangement, however, all five individual probes must always be connected simultaneously, and the attachment of further probes is not possible.